The present invention relates to a noise reduction system for a passenger compartment of an automotive vehicle by positively generating a sound to cancel the vehicle internal noise.
There have been proposed several techniques for reducing the noise sound in the passenger compartment by producing a canceling sound from a sound source disposed in the passenger compartment. The canceling sound has the same amplitude as the noise sound, but has a reversed phase thereto.
As a recent example, Japanese application laid open No. 1991-204354 discloses a vehicle internal noise reduction technique for reducing a noise sound by using a LMS (Least Means Square) algorithm (a theory for obtaining a filter coefficient by approximating it to a means square error in order to simplify a formula, utilizing that a filter correction formula is a recursive expression) or by employing a MEFX-LMS (Multiple Error Filtered X-LMS) algorithm. This technique has already been put to a practical use in some of vehicles. Commonly, an internal noise reduction system using this LMS algorithm is composed in such a way that: vibration noise source signal (primary source) is detected from an engine, the primary source is synthesized with a filter coefficient of an adaptive filter into a canceling sound, the canceling sound is generated from a speaker to cancel a noise sound in the passenger compartment; the noise sound reduced by the canceling sound is detected as an error signal by a microphone disposed at a noise receiving point; and based on the detected error signal and a compensation signal synthesized with a predetermined coefficient a filter coefficient of the adaptive filter is updated by the LMS algorithm so as to optimize the reduced noise sound at the noise receiving point.
It is known that an effective way for reducing an internal noise by producing a canceling sound is to coincide the direction from which the canceling sound comes with the one from which a vibration noise comes. That is to say, as indicated in FIGS. 5(a), (b), (c), (d) and (e), in case where the canceling sound comes from the same direction as the vibration noise, both are canceled each other at any position, providing that a noise sound and a canceling sound are plane waves having the same amplitude, the same frequency and a reversed phase to each other. However, on the other hand, in case where the canceling sound comes from an opposite direction to the vibration noise as shown in FIGS. 6(a), (b), (c), (d) and (e), the canceling sound cancels the vibration noise at the position of n.lambda./2 (for example, positions X.sub.a and X.sub.b), but at the position of (1+2n) .lambda./4 (for example, a position X.sub.c, mid-point of Xa and Xb) the vibration noise is interfered with the canceling sound and as a result of this interference it is amplified on the contrary (a relationship of the standing wave), where n is integers and .lambda. is a wave length. Especially, a noise reduction system using the LMS algorithm, among others, the MEFX-LMS algorithm has plural speakers from which canceling sounds are generated to cancel noise sounds at plural positions where a microphone is placed and plural independent control circuits for making control processes individually, therefore it happens that internal noise sounds changing rapidly according to operating conditions of engine could be reduced at a position where a microphone is located but could not at other positions away from the microphone. Further, depending on an operational condition of engine, the noise sounds may be amplified and get worse than in a case where no noise reduction control is performed.